This invention relates to a controlled d-c power supply for producing d-c voltage, at a selected desired amplitude level, from applied a-c line voltage, while at the same time protecting the d-c power supply against undesired line voltage fluctuations that may otherwise preclude accurate control over the d-c amplitude.
A well-known arrangement for developing an adjustable amplitude d-c voltage from a-c line voltage comprises an SCR rectifier bridge having a plurality of SCR's whose conduction angles are controlled in order to maintain a desired d-c magnitude. The greater the phase angle or time delay between the start of a half cycle of the a-c line voltage and the firing of the SCR's into conduction, the less the conduction angle and the less alternating current that will be rectified, thereby providing less d-c voltage across the output of the rectifier bridge. Depending on the load circuits to be driven by the bridge, it may or may not be followed by a low-pass filter (usually a series choke and a shunt capacitor) to eliminate the ripple component and smooth out the d-c voltage. For example, if the d-c voltage drives a d-c motor, it need not be filtered. On the other hand, filtering is preferred when the d-c voltage energizes an inverter.
Triggering the SCR's into conduction is accomplished by gating pulses applied to their gates, and the timing of these gating pulses may be determined by comparing, during each half cycle of the a-c line voltage, a ramp-shaped pulse with an error voltage representing the difference between the actual and desired d-c magnitudes. The ramp-shaped pulses may be developed from the a-c line voltage, but unfortunately line voltage fluctuations deleteriously affect those pulses (varying their widths) and cause improper timing of the gating pulses with resultant erroneous operation of the SCR bridge. Hence, the reliability and performance of this prior d-c power system suffers when the a-c line voltage deviates from its normal amplitude, as a consequence of which the d-c voltage cannot be regulated and held at a desired amplitude level.
The present invention overcomes this problem and provides an improved controlled d-c power supply which is immune to power line voltage variations and is highly efficient and reliable even in the presence of widely fluctuating line voltage.